The present invention relates, in general, to a frequency converter having an intermediate circuit without any capacitors and having a power supply device for supply of power to the electronics for this converter.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
A frequency converter of a type involved here has a slim intermediate circuit which is slim enough to eliminate the need for an intermediate circuit capacitor. A topology of a frequency converter with intermediate circuit without any capacitors is disclosed in the publication entitled “Fundamental Frequency Front End Converter (F3E)—a DC-link drive converter without electrolyte capacitor”, by Kurt Göpfrich, Dr. Rebbereh and Dr. Sack, printed in the Conference Proceedings PCIM 2003, Nürnberg, May 2003, and illustrated in FIG. 2. The F3E converter has a line-side converter 2 which, in addition to diodes D1 to D6, includes semiconductor switches T1 to T6 which can be switched off and are each connected electrically in parallel with the corresponding diodes D1 to D6. On the input side, the F3E converter has a line filter 20. In order to ensure that the electronics for this F3E converter can still be supplied with a supply voltage UV during brief power-line failures of the feeding mains, a buffered power supply device 12 is provided. It would also be conceivable to provide a power supply connected to the feeding mains. In the event of a power-line failure, the supply voltage UV at the two output connections would then collapse. Without adequate supply voltage UV, the electronics, and thus the F3E converter, will switch off.
In conventional frequency converters with a voltage intermediate circuit having at least one electrolytic capacitor as intermediate circuit capacitor, so-called “kinetic buffering (KIP)” is applied for bridging power-line failures. In this way, the drive that includes the frequency converter and the motor reaches its nominal rotation speed very quickly after the power supply has been restored. If no such option is provided in the frequency converter, the drive switches off, resulting in a relatively long restarting time for operation, since not only the signal processing for the frequency converter has to be started up again and reinitialized, but the motor must also be energized again and the rotation speed may have to be determined again.
In the “kinetic buffering” operating mode, when encountering a power-line failure, the drive is operated far enough in the generator range, i.e. braked, so that the mechanical (kinetic) energy of the motor and the connected process machine is able to cover the losses in the motor and the converter. This is realized with the aid of a regulator, which regulates the intermediate circuit voltage to a fixed value, for example 80% of its rated value. The manipulated variable is the torque nominal value or a supplement to the nominal rotational speed value in the case of field-oriented regulation, or a supplement to the frequency nominal value in the case of drives with U/f characteristic control. The voltage supply for the signal processing for the converter is either produced separately from a reliable source, or produced from the DC voltage intermediate circuit. Thus, signal processing and regulation remain active, so that the motor remains excited and is accelerated to its nominal rotation speed again immediately after the line voltage returns.
A precondition for “kinetic buffering” is that the intermediate circuit capacitor in the voltage intermediate circuit of a frequency converter is sufficiently large to enable a buffering of the intermediate circuit currents which occur in one switching period and are different in sign. If this precondition is satisfied, “kinetic buffering” is effective.
In a frequency converter configured as line-side converter having a diode rectifier, the voltage intermediate circuit requires an intermediate circuit capacitor which satisfies the abovementioned precondition.
In converters that have no intermediate circuit capacitors, or only extremely small intermediate circuit capacitors, application of “kinetic buffering” has not been possible. The capacitors which are required for continued operation are either not present or are on the line side in the case of the afore-described converters.
German Offenlegungsschrift DE 101 35 286 A1 discloses a method and an apparatus for bridging brief power-line failures in the case of a matrix converter. The matrix converter, which has a filter on the line side, can be connected to a feeding mains by means of a switch unit. When a power-line failure is detected, the matrix converter is disconnected from the feeding mains without delay and changes to a buffer mode by regulating a determined actual capacitor voltage space vector at a predetermined space vector. When the power is restored, the actual capacitor space vector is synchronized. Once the matrix converter has been synchronized, it is again connected to the feeding mains. The upstream disposition of a switch unit which has to switch very quickly makes it possible to use the capacitors in the line-side filter of the matrix converter as energy storage capacitors for “kinetic buffering” operating mode. As a result of the disconnection of the faulty power supply, these capacitors can be used as energy storage capacitors. Precondition for implementing “kinetic buffering” in a matrix converter is thus the presence of a high-speed line-side switch unit. A further drawback is the complexity of the regulation for line synchronization.
It would therefore be desirable and advantageous to provide an improved frequency converter to obviate prior art shortcomings and to allow use of “kinetic buffering” in a simple and yet reliable manner.